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Medical scientists think they have uncovered which portion of the brain assists people in ignoring distractions, according to a new study published in Nature. The study might help them to understand how defects in the thalamus could underlie symptoms witnessed in individuals with autism, schizophrenia and attention-deficit hyperactivity disorder.
Thirty years ago, Dr. Francis Crick proposed that the thalamus portion of the brain shines a light on regions of the cortex. This readies these regions to perform a task and leaves the remainder of the brain’s circuits in darkness and inactivity.
Senior study author Dr. Michael Halassa, PhD, from NY University’s Lagone Medical Center, explains that people only use a very small portion of incoming sensory stimuli to guide their behavior. The rest of the information is effectively filtered because it is unimportant.
In many neurological disorders, the filtering ability can be broken, which leads to a lack of control over sensory input so that the brain becomes overwhelmed with information.
Neuroscientists have long thought the prefrontal cortex is an area located at the front portion of the brain, selects what information to focus on, but how this occurs is a mystery.
One thought is that the neurons in the prefrontal cortex send out signals to the cells in the sensory cortices, located on the outer portions of the brain.
Dr. Halassa’s team believes that prefrontal cortex neurons could send signals to inhibit thalamic reticular nucleus cells located deeply in the brain. To find out more, he and his team designed a test that challenged mice to focus and ignore environmental distractions.
The mice were trained to use either a light or sound to uncover which of the two doors hid a food reward. Before each choice, the mice would hear a noise telling them to anticipate a light or the sound that would lead them to the right door. The mice had to use the proper hint and somehow ignore the irrelevant distraction to get the reward.
The research team used genetically modified mice in which specific neurons could be triggered or repressed with rays of light.
The mice made more mistakes when neurons in the prefrontal cortex were silenced during the anticipation of their cue. They picked the wrong door in response to the light or sound cue, which implied they could not concentrated with these neurons were silenced.
In contrast, when the neurons in the prefrontal cortex are silenced, the portion of the brain that processes visual data, at the moment of expectation, has no impact on attention.
The mice picked the right choice for a reward in response to a light clue. Contrary to what was previously believed, the connections between the prefrontal cortex and sensory cortical neurons would not appear to be involved with this sort of situation.
When the mice needed to focus on the light, activity levels in the visual TRN cells increased. When this circuit is turned off, it has the exact opposite effect. Instead of focusing on the light, mice now struggled to pay attention to the sound.
Scientists concluded that in order for environmental distractions to be prevented, the prefrontal cortex and thalamus areas of the brain must interact with one another. Dr. James Gnadt, PhD says, “We are constantly bombarded by information from our surroundings. This study shows how the circuits of the brain might decide which sensations to pay attention to.”
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